In general terms, the energy distribution across the cross section of a laser beam, at least in the lowest type of emission, has a gaussian appearance, which is in any case variable across the cross section of the beam.
However, in order to obtain, for example, an optimum annealing of amorphous silicon, it is necessary to apply a laser beam having a homogeneous and substantially uniform energy distribution.
This homogeneity is particularly required when the sample to be treated by laser is of large size (as described in the French Patent Application entitled "Device and method for laser surface treatment" filed in the name of the Applicant on Aug. 11, 1995, under the number 95 09778, and is incorporated herein by reference. It is also required when the incident laser beam is a combination of several laser beams coming from a line of laser units arranged in parallel and/or in series (as described in the French Patent Application entitled "Method and device for controlling a laser source with several laser units for optimizing laser surface treatment", also filed in the name of the Applicant on Aug. 11, 1995, under the number 95 09780, and is incorporated herein by reference.
Means for homogenizing a laser beam are already known.
For example, the document EP-A-0 266 120 or the document U.S. Pat. No. 5,253,110 describes a homogenizer which comprises a matrix of contiguous front lenses, disposed in m rows and n columns, perpendicularly to the direction of propagation of the laser beam to be treated, and able to divide the said laser beam into m.times.n laser beams each having a substantially geometrical uniform cross section, and a substantially homogeneous energy distribution.
In the document EP-A-0 266 120, at least one convergent collection lens, disposed perpendicularly to the direction of propagation of the light beam, downstream of the front lenses in the direction of travel of the laser beam, focuses, in a given area of a chosen plane, the beams coming from the front lenses.
In the documents referred to above, the matrix of lenses comes from a single piece machined to the format of the said matrix. It gives a plurality of laser beams each having a uniform and substantially identical intensity.
The drawback of such a matrix of lenses, formed of a Single piece, is that it makes it necessary to change the entire matrix when only one of the lenses is defective. In addition, the format of the matrix and the number of lenses are fixed. Such a matrix can, therefore, be suited only to a single size of laser beam to be homogenized.
Lenses assembled by bonding are known (CN-A-1 052 956), but this assembly does not deal with all light beams, owing to the glue disposed on the whole lateral surface of the faces of the lenses placed edge to edge. The adhesive is a cause of loss in the light transmission.
Furthermore, it can be destroyed by the power of the incident laser beam which irradiates it, lose its bonding properties or cause damage to the lenses.
Lenses are also known which are assembled individually by clamping in housings (DE-A-3 634 847), but this assembly also interferes with the path of the light beams, owing to the gaps separating the lenses.